diff --git a/.gitea/workflows/BuildAndroid.yml b/.gitea/workflows/BuildAndroid.yml
index da26ee0..983b5d4 100644
--- a/.gitea/workflows/BuildAndroid.yml
+++ b/.gitea/workflows/BuildAndroid.yml
@@ -29,11 +29,18 @@ jobs:
       - name: Checkout
         uses: https://github.com/actions/checkout@v4
 
-      # The flutter tool runs git inside the workspace, which the container
-      # user does not own after the checkout.
+      # The flutter tool runs git inside the workspace and inside its own sdk,
+      # and git refuses to work in directories it considers to belong to
+      # somebody else.
       - name: Trust the workspace
         run: git config --global --add safe.directory '*'
 
+      # Everything the flutter download below needs.
+      - name: Install the download tools
+        run: |
+          apt-get update
+          apt-get install -y --no-install-recommends ca-certificates curl xz-utils
+
       - name: Set up Java
         uses: https://github.com/actions/setup-java@v4
         with:
@@ -47,12 +54,19 @@ jobs:
       - name: Set up the Android sdk
         uses: https://github.com/android-actions/setup-android@v3
 
+      # `subosito/flutter-action` cannot be used: it caches the sdk through
+      # `actions/cache@v5`, and the act-runner of this gitea instance only
+      # supports actions up to node20 ("runs.using ... got node24"). Unpacking
+      # the official archive does the same thing and pins the version exactly,
+      # just like `ac_web/Dockerfile` does.
       - name: Set up Flutter
-        uses: https://github.com/subosito/flutter-action@v2
-        with:
-          flutter-version: ${{ env.FLUTTER_VERSION }}
-          channel: stable
-          cache: true
+        run: |
+          curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_${{ env.FLUTTER_VERSION }}-stable.tar.xz" \
+            | tar -xJ -C /opt
+          echo "/opt/flutter/bin" >> "$GITHUB_PATH"
+
+      - name: Show the flutter version
+        run: flutter --version
 
       - name: Install dependencies
         run: flutter pub get
diff --git a/.gitea/workflows/BuildLinux.yml b/.gitea/workflows/BuildLinux.yml
index 86ca983..0089974 100644
--- a/.gitea/workflows/BuildLinux.yml
+++ b/.gitea/workflows/BuildLinux.yml
@@ -28,8 +28,9 @@ jobs:
       - name: Checkout
         uses: https://github.com/actions/checkout@v4
 
-      # The flutter tool runs git inside the workspace, which the container
-      # user does not own after the checkout.
+      # The flutter tool runs git inside the workspace and inside its own sdk,
+      # and git refuses to work in directories it considers to belong to
+      # somebody else.
       - name: Trust the workspace
         run: git config --global --add safe.directory '*'
 
@@ -42,12 +43,19 @@ jobs:
             libgtk-3-dev liblzma-dev libstdc++-12-dev \
             curl unzip xz-utils
 
+      # `subosito/flutter-action` cannot be used: it caches the sdk through
+      # `actions/cache@v5`, and the act-runner of this gitea instance only
+      # supports actions up to node20 ("runs.using ... got node24"). Unpacking
+      # the official archive does the same thing and pins the version exactly,
+      # just like `ac_web/Dockerfile` does.
       - name: Set up Flutter
-        uses: https://github.com/subosito/flutter-action@v2
-        with:
-          flutter-version: ${{ env.FLUTTER_VERSION }}
-          channel: stable
-          cache: true
+        run: |
+          curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_${{ env.FLUTTER_VERSION }}-stable.tar.xz" \
+            | tar -xJ -C /opt
+          echo "/opt/flutter/bin" >> "$GITHUB_PATH"
+
+      - name: Show the flutter version
+        run: flutter --version
 
       - name: Install dependencies
         run: flutter pub get
diff --git a/CLAUDE.md b/CLAUDE.md
index 550a298..8caed07 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -44,6 +44,7 @@ Widget tests run against a mocked backend: build a `MockClient` that answers by
 
 Notes that are easy to get wrong:
 
+- **The act-runner of the gitea instance only supports actions up to `node20`.** Anything newer fails with `The runs.using key in action.yml must be one of: [...], got node24`, which is why the Flutter sdk is unpacked from the official archive instead of through `subosito/flutter-action` (that one caches through `actions/cache@v5`). Check the `runs.using` of an action before adding it.
 - The Flutter version is pinned in three places (`FLUTTER_VERSION` in both build workflows, `ARG FLUTTER_VERSION` in `ac_web/Dockerfile`) and must be kept in sync.
 - Steps run with `working-directory: ac_web`, but `upload-artifact` paths are relative to the repository root.
 - The workflows are read from the tag the release points at, so they have to be committed **before** the release is created.